1. Field of the Invention
The present invention relates to a liquid supply apparatus for supplying a liquid to a discharge head for discharging a liquid such as ink. The present invention particularly relates to a liquid supply apparatus and an ink jet recording apparatus in which a liquid, supplied from a main tank containing the liquid, is stored in a sub tank, from which the liquid is supplied to a discharge head.
2. Related Background Art
In a known ink jet recording apparatus, an interior of a nozzle, having a discharge port for discharging ink in a recording head, is maintained at a negative pressure. This is for forming a meniscus of the ink within the nozzle, in order to avoid ink leakage from the nozzle and air intrusion from the exterior into the nozzle.
In such known ink jet recording apparatus, the following component is known for generating a meniscus in the discharge port of the recording head. This component includes a main tank for containing ink and a sub tank for storing the ink supplied from the main tank and for supplying a recording head with the ink, in which the sub tank is positioned lower than the recording head in the vertical direction.
In such system, an ink supply from the sub tank to the recording head is executed utilizing a water head difference between the sub tank and the recording head. In the known ink supply apparatus, a chicken feed method is known as a simple structure for executing the ink supply from the main tank to the sub tank. A chicken feed configuration involves a container having an opening which is opened upon liquid being filled to the top of the container. The ink supply structure by the chicken feed is disclosed in FIG. 3 of U.S. Pat. No. 6,024,442 and FIG. 3 of U.S. Pat. No. 6,338,552. A prior ink supply apparatus of the chicken feed method, disclosed in Japanese Patent Application Laid-open No. H03-247460, is shown in FIG. 1. An ink-containing air-tight replaceable main tank is mounted on a tank mounting portion in the recording apparatus, whereby the ink is supplied from the main tank to the sub tank through an ink supply tube. However, in the ink supply apparatus of such structure, the main tank is provided, at an ink supply aperture thereof, with a spring-biased valve which is to be opened upon mounting in the recording apparatus. Such valve may be opened, for example, by an impact when the main tank is dropped, thereby causing ink leakage.
For preventing such ink leakage, the following structure is already known. As shown in FIG. 7, an ink supply apparatus 700 is equipped with an ink supply needle 703 and an air introducing needle 704. The ink supply needle 703 is provided for ink supply from an ink-containing air-tight replaceable main tank 701 to a sub tank 702, and the air introducing needle 704 is provided for introducing air into the main tank 701, when the liquid is replaced by air. The ink supply apparatus is equipped with a supply base 705, having these hollow needles. In the main tank 701, an aperture of a connecting part to be connected to the supply base 705 is closed by a rubber stopper 706. At the connection with the supply base 705, the ink supply needle 703 and the air introducing needle 704 pierce the rubber stopper 706, thereby communicating with the interior of the main tank 701. Thus an ink supply from the main tank 701 is executed through these two needles to the supply base 705, from which the ink is supplied to the sub tank 702 through the ink supply tube 707.
The main tank 701 is provided, at the connecting part thereof, with the elastic rubber stopper 706. When the main tank 701 is extracted from the ink supply needle 703 and the air introducing needle 704 of the supply base 705, the piercing holes are closed air-tight by the elasticity of the rubber stopper 706. It is thus possible to satisfactorily secure the air-tightness in the connecting part of the main tank 701, thereby sufficiently preventing ink scattering from the connecting part in case the main tank 701 is dropped.
It is necessary to prevent the rubber stopper 706 to be pierced by the ink supply needle 703 and the air introducing needle 704 from breaking by cracking at the piercing. To avoid breakage, these two hollow needles are restricted at where the internal diameter increases. The internal diameter of the ink supply needle 703 and the air introducing needle 704 is usually selected at about 1.6 mm.
Also Japanese Patent Application Laid-open No. 2002-234180 discloses a constitution of chicken feed of connecting an air-tight replaceable main tank by a hollow tube at a connecting part, thereby securing a water head difference with the recording head. Also, Japanese Patent Application Laid-open No. 2004-142442 discloses a chicken feed structure utilizing plural communicating tubes in the ink supply between a first ink tank and a closed second ink tank.
Such known chicken feed apparatuses utilizing a hollow air introducing needle, in case of employing a narrow air introducing needle with an internal diameter of about 1.6 mm, results in the following drawback. An ink meniscus is often formed in the interior or the air introducing needle which communicates to the external air or at a lower end portion of such air introducing needle, and such meniscus is in an uncertain state. In such prior liquid supply apparatus, a negative pressure generated by the water head difference in the closed main tank may become smaller than a meniscus force, thereby inhibiting the ink supply from the main tank to the sub tank and deteriorating the recording state by the recording head.
The ink supply structure by the chicken feed system disclosed in Japanese Patent Application Laid-open No. 2002-234180 does not show a specific constitution for maintaining a stable water head. Japanese Patent Application Laid-open No. 2004-142442 discloses a constitution of providing, for ink supply between a first ink tank and a closed second ink tank, plural communicating tubes in which the communicating paths have different internal diameters. In such constitution, however, the first tank is opened to the air, while the second ink tank is made air-tight. Therefore, what flows in the communicating tube varies depending on the condition of ink supply, so that each of the plural communicating tubes becomes an air flow path or a liquid flow path. Also, the second ink tank is provide therein with a spring member for generating a negative pressure. It is not so constructed to maintain a stable liquid level therein at a constant position, and the ink level therein is variable.
As described above, the known ink supply apparatus has the drawback in that the negative pressure fluctuates depending on an ink amount remaining in the main tank, whereby an ink liquid level in the sub tank tends to unstably fluctuate. Also, in the known ink supply apparatus, the ink amount remaining in the sub tank fluctuates when the ink in the main tank is exhausted or when the main tank is detached. It is therefore difficult, when the ink in the main tank is no longer available, to ensure a recording time or a number of recording sheets for continuous recording with the ink remaining in the sub tank.